Strange Knocking Noise...

[chris73]

Been to Silverstone today to the Ferrari 60th, excellent day except for...

On the return trip a very distinct knocking sound (like i have a bit of scaffolding knocking against something metal and solid quite hard) which you could feel through the seat - only when totally off the throttle, less pronounced with the clutch down. Not linked to engine speed, engine is fine (revs well, pulls (very!) well). Gearbox seems fine, every gear as usual through the box. Braking from 80mph everything fine, down to to 45-50mph then you hear and feel a knock... knock.... knock as you slow to 20-30mph. As you blip throttle to come down through the box you get no knock...

Pulled over, checked all suspension mountings, engine mounts, exhausts, wheels, calipers and generally had a damn good look around and scratched head. Got back in muttering and completed our journey (braking with the clutch down) sucessfully. As long as i dont slow down, its fine!!

HELP - i really dont know what is going on - am still quite new to the car (8 weeks) and this threatens Le Mans trip on Thursday. I am in Bristol if anyone knowns anyone, garage, bloke whatever would be much appreciated.

[ultimaCZ]

Did you have air-con switched on or off?
The A/C can do very strange knocking noises if wrongly filled and when hot. Had that once and thought its the engine
Little chance of that being the cause, but worth asking...

Edited by ultimaCZ on Sunday 10th June 23:57

[ezakimak]

When you say engine mounts, there was a thread a while back where it was discovered that some parts on the engine mounts may need to be ground off to avoid contact with the chassis. Once the engine is in I don’t think you would be able to see it.

[Thevet]

Having just been playing at modifying engine mounts, I can't believe there could be enough play to allow a "knock", and there is also a rubber damping section to the mounts to absorb movement.

[crafty]

Thinking external stuff first (those problems are much cheaper).... Have you by chance bent the chassis tube under the adaptor plate? If that were bent up it might hit the adaptor plate and the engine would knock against that.

Another source of irritating knocking are the Jehovah's Witnesses... maybe you ran one over.

Edited by crafty on Monday 11th June 08:49

[chris73]

Thanks Guys, air con was on all the time.

Checked for Jehovah body parts underneath and none found (although i know they like to hide and try again)...

[crafty]

Dry suspension bushes can make a banging sound. When I was with Jeff on Power Tour there was a banging sound and I was convinced it was something hitting the chassis. He was adament that it was dry suspension bushes. We sprayed em with silicone spray and the banging went away.

[ultimaCZ]

chris73 said:

Thanks Guys, air con was on all the time.

Simple check then, try to drive with and without A/C. You might be in for a surprise... (but A/C must be running for some time, this problem appears only when the system is hot)

[chris73]

Went out on run with Air-Con off - no difference. Noticed left hand exhaust pipe sticking out more than it should, on closer examination one of the rubber mounting bushes has split causing the silencer->tail pipe clamp to bang on the chassis as the engine moves when you lift off. £3.64 fix - phew, am very 'appy!

Thanks for advice all, See you in Le Mans...

[LuckyP]

chris73 said:

Went out on run with Air-Con off - no difference. Noticed left hand exhaust pipe sticking out more than it should, on closer examination one of the rubber mounting bushes has split causing the silencer->tail pipe clamp to bang on the chassis as the engine moves when you lift off. £3.64 fix - phew, am very 'appy!

Thanks for advice all, See you in Le Mans...

Good news chap - I love cheap fixes.....wonder if I'll ever have one!!??

Enjoy LM